It sounds like you have a T-Mobile device that had been previously upgraded to an ATT OS. Pretty common, and nothing wrong with that. Upgrading your OS shouldn't do any harm to your unlocked status.

Incorrect. Changing your OS will never re brand your phone with a different carriers logo. You simply have an AT&T phone that someone has used a t-mobile replacement part on. Once your phone is unlocked changing your OS will not lock it again. Ndub33 was right telling you that you might not get all your servive books. What carrier are you using in India?